The sillenites Bi12MO20 (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2) have been recognised as promising bulk materials for electronic applications because their dielectric properties (5.5 GHz: ε = 37.4 - 41, τf = -19 – (-84) ppm/K, Q x f = 800 - 8100 and Tsin = 680 - …